Different sized waveguides coupled by a narrow tapered dielectric rod



Aug. 18, 1964 P. J. B. CLARRICOATS Q 3,145,356

DIFFERENT SIZED WAVEGUIDES COUPLED BY A NARROW TAPERED DIELECTRIC RODFiled Oct. 6. 1961 United States Patent ce 3,145,356 DIFFERENT SIZEDWAVEGUIDES COUPLED BY A NARROW TAPERED DIELECTRIC ROD Peter John BellClarricoats, Belfast, Northern Ireland,

assignor to National Research Development Corporation, London, England,a British corporation Filed Oct. 6, 1961, Ser. No. 143,499 Claimspriority, application Great Britain Oct. 11, 1960 3 Claims. (Cl. 333-34)This invention relates to methods of coupling two electromagneticwaveguides of dilfering cross-section without substantial impedancemis-match so that the apparatus may be used over a broad frequency band.

When two Waveguides of different cross-section and therefore differentcharacteristic impedance are joined together, the discontinuity in theboundary conditions causes reflections in an electromagnetic wavetravelling along the guides. Whereas it is relatively straight-forwardto match the two guides for any one frequency so that there is noreflection at that frequency, it is not easy to do this over a broadband of frequency. Known methods of doing this include the tapering ofthe waveguide dimension over a distance of several wavelengths, theforming of stepped transitions at the junction, and the inserting of adielectric transformer in one of the waveguides to match thecharacteristic impedances of the two waveguides. The first methodrequires accurately made taper transitions, while in the latter twomethods the dimensions of the apparatus are critical and need to becalculated separately for each individual case.

It is an object of this invention to provide a simplified device whichwill couple together two wave-guides of different cross-section withoutsubstantial impedance mismatch.

It has now been found that if an electromagnetic Wave is propagatedalong a dielectric rod in free space, and if the rod dimensions aresuitably chosen, virtually all the energy associated with the wavepasses along within the rod. This being so, it is obvious that theintroduction of metallic bounding surfaces in the free space outside thedielectric rod will have virtually no effect on the wave, provided thatthese surfaces are not too near the dielectric. Hence, if a dielectricmember is supported within a waveguide out of contact with the wallsthereof when an electromagnetic wave is propagated along the Waveguide,virtually all the energy associated with the Wave will be concentratedin the dielectric. This means that the cross section of the boundingwaveguide may be changed abruptly without appreciably affecting the wavewithin the dielectric, or, in other words that the dielectric rod actsas a guide for the waves as they pass through the region of abruptwaveguide change. The energy will be confined within the rod providedonly that the dimensions of the rod and the permittivity of thedielectric be so chosen that the wavelength in the rod, A is less thanthe wavelength in free space, A 7

Since the concentration of energy within a dielectric rod can withappropriate choice of rod dimensions be made broad band, it follows thatsuch a rod may be used to provide a broad band junction between twowaveguides of differing size and/ or shape. It is of course necessary toensure that the energy in the waveguide is transferred to the dielectricrod with adequate efiiciency, and this may be achieved by any of theknown methods, such as by the 3,145,356 Patented Aug. 18, 1964 taperingof the rod over a distance of as many wavelengths as is possible, or,Where a more compact form of junction is required, by the formation ofstepped transitions of known form. The only requirements of thedielectric rod are that its dimensions and permittivity should besufiiciently large for k to be less than M, while the cross-section ofthe rod should not approach too closely that of the smaller waveguide.Although the cross-sectional shape of the rod is not a criticalconsideration, it is preferred to use a rod of the same shape as that ofthe coupled waveguides.

One embodiment of the present invention will now be described by Way ofexample and with reference to the accompanying diagram in which thesingle figure represents in longitudinal cross-section a waveguidejunction between two waveguides of different dimensions.

In the drawing 1 and 2 are two circular waveguides which are joinedtogether end to end. 3 is a dielectric rod made of, for example,polystyrene, which is tapered to points at 4 and 5, and which issupported centrally in the guide at 6 in a block of a substance, such asfoamed polystyrene, which is substantially transparent to microwaveradiation but mechanically quite rigid. The diameter of the dielectricrod is large enough for k to be less than M but less than that of thesmaller waveguide; for optimum working it should be about equal to themean of the value for which a i and the smaller waveguide diameter.

This invention may also be used to join two waveguides of differentshape e.g. rectangular and circular when a rod shape intermediatebetween the shapes of the two Waveguides may be used, e.g. polygonal inthe case of rectangular and circular. Alternatively a rod of varyingcrosssection may be used, provided that any change in the shape of therod be introduced gradually in order not to cause additionalreflections.

When the two waveguides to be joined are of grossly differingcross-section, a high permittivity material may be used for thedielectric rod, in order that the rod should be small enough to becompatible With the smaller waveguide, while still satisfying thecondition that be less than ho.

Furthermore while the arrangement illustrated employs a rod havingtapered ends, a rod having stepped transi tions at its ends could beused, or else a series of rods of differing diameters andpermittivities.

I claim:

1. Apparatus for coupling together two electromagnetic waveguides ofdifferent cross-sectional dimensions comprising a rod of dielectricmaterial extending between the two waveguides, the rod having suchcross-section and permittivity that the wavelength of electromagneticenergy propagated in the rod is less than the wavelength of such energyin free space and in which the cross-section of the rod is sufficientlylarge to permit substantially the whole of the energy of a wavetransmitted across the coupling to be concentrated in the rod, withoutbeing so large relative to the cross-section of the smaller waveguide asto alloW the walls of said smaller waveguide to appreciably affect thepropagation characteristics of the energy concentrated in the rod,whereby wave energy may be propagated through said coupling in eithersense.

2. The arrangement as claimed in claim 1 in which the ends of the rodare tapered.

3. The arrangement as claimed in claim 1 in which the cross-section ofthe rod is about equal to the mean be- 1 3 4 tween the cross-section ofthe smaller Waveguide and that 2,503,549 Fox Apr. 11, 1950 cross-sectionof rod for which the Wavelength of energy 2,625,605 Chadler Jan. 13,I953 propagated therein would equal the wavelength in free 2,742,612Cohn Apr. 17, 1956 space. 2,802,994 Ober et al Aug. 13, 1957 5 2,906,974Reggia et al Sept. 29, 1959 References Cited in the file of this patent3,010,083 Weiss Nov. 21, 1961 UNITED STATES PATENTS OTHER REFERENCES2,207,845 WOHT July 1940 Weiss: IRE Transactions on Microwave Theory and2,292,496 Von Baeyer F" 1942 Techniques, January 1960, pages 120-121.

2,422,191 Fox June 17, 1947 10

1. APPARATUS FOR COUPLING TOGETHER TWO ELECTROMAGNETIC WAVEGUIDES OFDIFFERENT CROSS-SECTIONAL DIMENSIONS COMPRISING A ROD OF DIELECTRICMATERIAL EXTENDING BETWEEN THE TWO WAVEGUIDES, THE ROD HAVING SUCHCROSS-SECTION AND PERMITTIVITY THAT THE WAVELENGTH OF ELECTROMAGNETICENERGY PROPAGATED IN THE ROD IS LESS THAN THE WAVELENGTH OF SUCH ENERGYIN FREE SPACE AND IN WHICH THE CROSS-SECTION OF THE ROD IS SUFFICIENTLYLARGE TO PERMIT SUBSTANTIALLY THE WHOLE OF THE ENERGY OF A WAVETRANSMITTED ACROSS THE COUPLING TO BE CONCENTRATED IN THE ROD, WITHOUTBEING SO LARGE RELATIVE TO THE CROSS-SECTION OF THE SMALLER WAVEGUIDE ASTO ALLOW THE WALLS OF SAID SMALLER WAVEGUIDE TO APPRECIABLY AFFECT THEPROPAGATION CHARACTERISTICS OF THE ENERGY CONCENTRATED IN THE ROD,WHEREBY WAVE ENERGY MAY BE PROPAGATED THROUGH SAID COUPLING IN EITHERSENSE.